The recovery of residual oil from oil-bearing subterranean formations or reservoirs utilizing as a drive fluid, or mobility buffer, an aqueous solution containing a polymer such as a partially hydrolyzed polyacrylamide has been shown to be both effective and economically feasible. While various techniques for the preparation of aqueous polymer solutions have been proposed, the solutions usually are prepared by forming an aqueous solution of a monomer in a vessel of suitable volume. The monomer may be in the form of a solid or a liquid. Exemplary of such a monomer is acrylamide which is available commercially both in solid form, or as a 50%, by weight, Cu.sup.++ inhibited aqueous solution. When employed in the latter form, the Cu.sup.++ inhibited solution advantageously is first passed through an ion exchange resin to remove the copper ions. The aqueous monomer solution in the vessel, whether the monomer is employed in the form of a solid or a liquid, will comprise from about 1% to about 10%, more or less, by weight, monomer. The monomer solution is then sparged with nitrogen to reduce the concentration of oxygen in the solution to a desired level. A polymerization initiator thereafter is introduced into the vessel containing the monomer solution, and polymerization of the monomer is allowed to proceed until complete. In the case of acrylamide, polymerization is usually completed in from 8 to 12 hours. The aqueous polymer solution is then pumped to a second vessel where a hydrolyzing agent such as a dilute aqueous solution of a monovalent alkali metal hydroxide exemplified by sodium hydroxide is introduced. The resulting reaction mixture of 1-2 wt. % polymer is retained in the second vessel for a period of 10 to 12 hours, or more, to enable partial hydrolyzation of the polyacrylamide to go to completion. The resulting aqueous solution of partially hydrolyzed polymer may then be transferred to a holding tank for temporary storage, or it may be further diluted for immediate injection into an input well.
The substantial time periods required in completing the polymerization of the monomer, and the subsequent partial hydrolyzation of the formed polymer, by the above described process derogate from its use in an on-site, semi-continuous, or continuous system for preparing partially hydrolyzed polymers of the type employed in the secondary and tertiary recovery of oil where a constant supply of large volumes of aqueous polymer solution is needed to successfully carry out the operation.